System and method for crystal growing

ABSTRACT

To reduce the heat input to the bottom of the crucible and to control heat extraction independently of heat input, a shield can be raised between a heating element and a crucible at a controlled speed as the crystal grows. Other steps could include moving the crucible, but this process can avoid having to move the crucible. A temperature gradient is produced by shielding only a portion of the heating element; for example, the bottom portion of a cylindrical element can be shielded to cause heat transfer to be less in the bottom of the crucible than at the top, thereby causing a stabilizing temperature gradient in the crucible.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of, and claims the benefit under 35U.S.C. §120 of U.S. patent application Ser. No. 11/875,078, filed Oct.19, 2007, pending; which is a divisional of U.S. patent application Ser.No. 11/212,027, filed Aug. 25, 2005 and entitled “System and Method forCrystal Growing,” now U.S. Pat. No. 7,344,596, the entire contents ofeach of which are incorporated herein by reference.

BACKGROUND

The systems and methods described here relate to systems and methods forproducing crystals.

Materials grown in single crystal form in an as-grown state aretypically referred to as boules, while materials in multicrystallineform are referred to as multicrystalline ingots. At times, boules andingots are collectively referred to as crystals. In this document, theterm “crystals” is intended to include at least boules and ingotscollectively, and, in some instances, boules and ingots may be referredto separately to demonstrate distinctions between single crystal andmulticrystalline materials.

For crystal growth in some systems it can be desirable to grow crystals,such as sapphire or silicon, from the bottom to the top of a cruciblethat holds a molten material. The bottom of the furnace should thereforebe cooler than the top, preferably with a stabilizing temperaturegradient that minimizes convection and avoids constitutionalsupercooling. The material in the crucible can solidify from the bottomto the top due to the vertical gradient. This process helps to achievedirectional solidification and thereby rejection of impurities to themelt for impurities having a segregation coefficient of less than 1(very rarely do impurities have a segregation coefficient greaterthan 1. The process thereby produces a purer solid. In case of silicon,the segregation coefficient of Fe is 10⁻⁶ and for refractory metals itis even less than 10⁻⁹; consequently, directional solidification can bean effective purification process. For reactions with the melt resultingin volatile product, the gases can be rejected upwardly through the meltso it can escape.

SUMMARY

To reduce the heat input to the bottom of the crucible and to controlheat extraction independently of heat input, a shield can be raisedbetween a heating element and a crucible at a controlled speed as thecrystal grows, preferably without moving the crucible, but this processcan be performed with a movable crucible. A temperature gradient isproduced by shielding only a portion of a heating element; for example,the bottom portion of a cylindrical element can be shielded to causeheat transfer from the heating element to be less to the bottom of thecrucible than at the top, thereby promoting controlled solidification ofthe charge in the crucible from the bottom upwards and causing astabilizing temperature gradient in the crucible. Other features andadvantages will become apparent from the following description,drawings, and claims

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1( a)-1(c) are cross-sectional views of a furnace used to producemulticrystalline ingots.

FIGS. 2( a)-2(b) are cross-sectional views of a furnace used to producesingle crystal boules.

DESCRIPTION

Crystal growth in a crucible is driven by heat flow. In the systems andmethods described here, both heat input and heat extraction arecontrolled. The heat input is controlled by heat transfer from a heatingelement to contents in the crucible, which depends on the difference intemperature between the heating element and the outside of the crucible.Heat extraction from a heat zone of a furnace can be controlled byincreasing heat extraction at the bottom of the furnace, e.g., bylowering insulation to increase heat loss to lower the temperature, orby using a heat exchanger to extract heat to produce a temperaturegradient in the heat zone.

One previously known method to reduce the temperature in the bottom ofthe crucible is to move a crucible out of a heat zone and into a coolerarea, e.g., by lowering the crucible out of the heat zone. Moving thecrucible can be difficult at high temperature and with a heavy crucible,and can cause vibrations and perturbations at the solid/liquidinterface.

To reduce the heat input to the bottom of the crucible and to controlheat extraction independently of heat input, a shield can be raisedbetween a heating element and a crucible at a controlled speed as thecrystal grows. Other steps could include moving the crucible, but thisprocess can avoid having to move the crucible. Heat input to the chargeis reduced by shielding only a portion of the heating element; forexample, the bottom portion of a cylindrical element can be shielded tocause the heat transfer from the heating element to be less to thebottom of the crucible, thereby promoting solidification near the bottomof the molten charge and promoting the solidification in a controlledmanner (directional solidification) by moving the heat shields upward ata controlled rate.

The heat extraction can be controlled independently of the heat input bylowering the insulation or by using a heat exchanger as currently usedis the Heat Exchanger Method (HEM) by Crystal Systems, Inc. With the HEMmethod, the heat input is decreased without moving the crucible and theheat extraction is independently controlled.

The HEM method of crystal growth is described in U.S. Pat. No. 3,898,051for single crystals such as sapphire, germanium, and III/V compounds;U.S. Pat. No. 4,256,530 for silicon crystal growth; U.S. Pat. No.4,840,699 for GaAs crystal growth; and in U.S. Pat. No. 3,653,432. Theseapproaches include independent control of temperature gradients in theliquid and the solid during crystal growth without moving the heat zone,crystal, or crucible. While this description focuses primarily on theHEM method, it is applicable to other techniques in which growth occursfrom the bottom to the top of a melt, such as Vertical Bridgman,Modified Bridgman, Thermal Gradient Freeze (TGF), and Thermal GradientTechnique (TGT).

In the HEM method, a nearly isothermal heat zone was designed in which ahigh temperature heat exchanger was inserted from the bottom of thechamber. When the charge is melted, a minimal flow of helium gas throughthe heat exchanger prevents the seed crystal from melting out. Thecharge is melted and the seed crystal is melted back by superheating themelt above its melting temperature. The helium flow through the heatexchanger is increased, thereby decreasing the heat exchangertemperature and/or by decreasing the furnace temperature to grow thecrystal. During most of the growth cycle, the temperature gradients inthe liquid are primarily controlled by the furnace temperature and thetemperature gradients in the solid by the heat exchanger temperature.These temperatures are controlled independently; hence, this methodindependently controls temperature gradients in the liquid and solidwithout the need to move the heat zone, crystal, or crucible. Thesolid-liquid interface is submerged and therefore any mechanical orthermal perturbations are damped out by surrounding liquid beforereaching the solid-liquid interface. It is not necessary to rotate thecrucible to minimize the effects of hot/cold spots in the heat zone,although it could be so rotated. After crystal growth, the crystal isstill in the heat zone so the furnace temperature can be reduced belowthe melting point of the material and the temperature gradient imposedby the helium flow through the heat exchanger can be reduced bydecreasing the heat flow. Under these conditions, the crystal can be insitu annealed to relieve solidification stresses and reduce defectdensity prior to controlled cooldown of the crystal.

This process has been used to produce large sapphire crystals up to 15inch (38 cm) diameter, titanium-doped sapphire crystals up to 8 inch (20cm) diameter, and GaAs crystals up to 4 inch (10 cm) diameter. When asquare cross-section crucible was used to contain the charge, squarecross-section crystals were produced even though the heat zone wascylindrical. This has been demonstrated with silicon.

For isotropic materials, such as silicon, single crystal growth may notbe necessary and in some applications multicrystalline ingots withcontrolled grain size and orientation are comparable in performance. Forexample, high-quality single crystal silicon is desired for mostsemiconductor devices for the microelectronics industry, but forphotovoltaic (PV) applications high-quality multicrystalline silicon canbe used for most applications. For this application, the cost and highvolume production are important, and a slight compromise in quality canbe tolerated. Therefore, the PV devices for terrestrial applicationstend to use large multicrystalline silicon ingots.

Referring to FIG. 1( a), a furnace 10 includes furnace insulation 26,and a crucible 12 containing a molten liquid 14. Crucible 12 sits on asupport block 16 (e.g., made of graphite), which is in contact with aheat exchanger 18 in the form of a support rod. Around the crucible 12are one or more heating elements 20. As shown here, a conventional tubeheat exchanger was replaced with a movable insulation pack 22 that canbe moved relative to block 16. This process can promote rapid growth.All the charge can be melted and insulation pack 22 under crucible 12 islowered (FIG. 1( b)) so that heat is radiated from the graphite block tothe cooler sections of the chamber. Under these conditions, a planarsolid-liquid interface can be generated and the orientation of the grainboundaries can be nearly vertical.

With this approach, large multicrystalline silicon ingots were producedwith centimeter-size grains, vertical orientation of the grainboundaries, and no impinging interfaces which resulted in producinghigh-efficiency solar cells comparable to those produced using singlecrystal silicon. In this process, the heat extraction took place throughthe entire bottom of the crucible. As the crucible size is increased,the heat extraction area increases as well.

In another process, the crucible was also lowered in the heat zoneduring the growth cycle to help grow taller multicrystalline siliconingots. After the growth was completed, the furnace temperature wasreduced below the melting point of silicon, and the crucible and theinsulation pack were moved back up to their original position to achievein situ annealing of the ingot prior to cooldown. This resulted inproducing ingots up to 69-cm square cross-section up to 300-kg at lowcost. The system can be used to produce ingots or boules of 300 kg andgreater.

Lowering the crucible in the heat zone promotes heat extraction, but theingot is not lowered beyond where the gradients on the solid increase toimpose stress on the ingot. The ingot in its lowered state is stillsubjected to heat transfer from the heating element to top of the ingot,which has to be removed by the heat extraction system. Therefore, largetemperature gradients can be generated by the high heat input and theheat extractions.

The process of FIGS. 2( a)-2(b) is similar to FIGS. 1( a)-1(c).

There are similarities in the two approaches of FIGS. 1( a)-1(c) and2(a)-2(b), with the main difference being in the shape of thesolid-liquid interface during growth, and that FIGS. 2( a)-2(b) may notinclude an insulation pack or may be of smaller size consistent with thesize of the seed crystal. For single crystal growth, a hemisphericalprofile is achieved to allow nucleation and growth off a small seedcrystal. For multicrystalline growth, a slightly convex nearly planargrowth interface covering most of the bottom of the crucible allowsformation of large grains with nearly vertical orientation of grainboundaries.

After the charge is melted under the controlled atmosphere desired forthe material for growth of multicrystalline ingots, the movableinsulation pack is lowered to promote heat extraction from the block andthe melt. In the case of single crystal boules, heat extraction ispromoted by lowering the smaller insulation pack and/or increasing theflow of helium gas through a heat exchanger without the insulation pack.

Referring to FIGS. 1( c) and 2(b), to sustain growth in both cases, amovable heat shield 24 positioned between the heating element and thecrucible can be moved upward so that the heat input to the charge isreduced as the heat shield is moved upward in the heat zone. As theshielding is raised the heat input is decreased and therefore reasonablegrowth rates are maintained without the requirement of considerably moreheat extraction.

The heat extraction at the bottom of the crucible can be increased bydecreasing either the heat exchanger temperature or by lowering theinsulation under the crucible before, after, or while the shielding israised. The rate of movement of the heat shields can be independentlycontrolled to control both heat input and heat extraction from thebottom of the crucible to achieve the most favorable crystal growthcondition. Under these conditions, a convex interface can be maintained.

Therefore, the temperature gradients in the liquid and the solid arereduced and higher quality crystals can be produced at a faster growthrate. An additional advantage is that larger diameter and talleringots/boules can be grown without introducing additional stress andcausing spurious nucleation.

After the solidification is complete, the furnace temperature can bereduced below the melting point of the material and the heat shield canbe lowered to its original position and heat exchanger turned off orinsulation under the crucible raised to remove temperature gradient forin situ annealing of the crystal. The crystal is then cooled to roomtemperature at a rate that does not introduce stress in the boule oringot.

In addition to the component shown in FIGS. 1( a)-1(c) and 2(a)-2(b),the system would also typically include a number of sensors, and wouldtypically include a controller, such as a microprocessor-based computeror some other method for controlling the movement of the shield orinsulation pack.

Having described certain embodiments, it should be apparent thatmodifications can be made without departing from the scope of theinvention as defined by the appended claims.

1. A system for growing a crystal from a liquid in a crucible, thesystem comprising: a support structure to support the crucible frombelow; at least one heating element to provide heat to the crucible; anda heat exchanger to extract heat from the support structure, wherein theheat exchanger includes insulation that is movable away from the supportstructure.
 2. The system of claim 1, wherein the support structureincludes a graphite support block.
 3. The system of claim 1, wherein theat least one heating element includes a plurality of electrical heatingelements positioned to the side of the crucible.
 4. The system of claim1, wherein the crystal is a multicrystalline ingot.
 5. The system ofclaim 1, wherein the crystal is a single crystal boule.
 6. The system ofclaim 1, wherein the crystal is silicon.
 7. The system of claim 1,wherein the support structure and crucible are configured to be kept ina stationary position while a crystal is grown, and wherein the heatexchanger is movable while the crystal is grown.
 8. The system of claim7, further comprising a controller for controlling the amount of heatprovided from the at least one heating element to the crucible, and forcontrolling the heat exchanger to cause the insulation to be moved to becloser to or further from the crucible while a crystal is grown.
 9. Amethod of growing a crystal from a liquid in a crucible, said methodcomprising: heating the crucible with at least one heating element; andextracting heat from the crucible using a heat exchanger in thermalcontact with the crucible, wherein the crucible is supported from belowby a support structure and wherein the heat exchanger includesinsulation that is movable away from the support structure.
 10. Themethod of claim 9, wherein the step of extracting heat from the crucibleincludes moving the insulation away from the support structure.
 11. Themethod of claim 10, wherein the insulation is lowered away from thesupport structure.
 12. The method of claim 9, wherein heating thecrucible includes operating a plurality of electric heating elementspositioned to the side of the crucible.
 13. The method of claim 9,wherein the support structure includes a graphite support block, theextracting including moving the insulation away from the graphitesupport block.
 14. The method of claim 9, wherein the support structureand crucible are not moved during the crystal growing process, andwherein the insulation is moved during the crystal growing process. 15.The method of claim 9, wherein the crystal is silicon.
 16. The method ofclaim 9, wherein the crystal is a multicrystalline ingot.
 17. The methodof claim 9, wherein the crystal is a single crystal boule.
 18. Themethod of claim 9, further comprising controlling the at least oneheating element to increase and decrease the heat provided to thecrucible, and further controlling the extracting by causing theinsulation to be moved to be closer to or further away from thecrucible.